Forward schema_path/target_class to linkml's delimited file loader

[Sigfried]

Summary

Forward schema_path and target_class through TsvFileLoader, CsvFileLoader, DataLoader, and get_file_loader() so they reach linkml's underlying TsvLoader / CsvLoader. This enables schema-aware type coercion for delimited files, preventing string-ranged and enum-ranged columns from being silently coerced to int/float.

Without these parameters, a column like subject_id containing numeric-looking strings (e.g., "00123") gets loaded as 123 (int) by pandas' default inference, losing the leading zero and breaking downstream lookups.

Background

linkml/linkml#3289 ("Make delimited file loader schema-aware to preserve string/enum columns") added schema-awareness to linkml's _DelimitedFileLoader. It was released in linkml v1.11.0. This PR exposes those parameters through linkml-map's loader API so users of linkml-map can benefit from the same fix.

Changes

• TsvFileLoader.__init__ / CsvFileLoader.__init__: accept optional schema_path and target_class
• iter_instances(): forward both params to the underlying linkml loader
• get_file_loader(): kwargs-only schema_path / target_class, passed only to TSV/CSV loader classes (other formats already do their own type handling)
• DataLoader.__init__: accept and store both params
• Tests: 11 new `TestSchemaAware*` tests in `tests/test_loaders/test_data_loader.py` covering string-range preservation, enum-range preservation, and the no-schema coercion fallback for each entry point (`TsvFileLoader`, `CsvFileLoader`, `get_file_loader`, `DataLoader`)

The change is additive — existing callers that don't pass the new params get current behavior. Annotations use PEP 604 `X | Y | None` syntax to match the rest of the file's style (no new `typing` imports needed).

Test plan

• 42 tests pass in `tests/test_loaders/test_data_loader.py` (31 existing + 11 new)
• Full `pytest` run: 831 passed, 4 skipped, 1 pre-existing failure (`test_graphviz_compiler` — fails on `main` too due to missing local `dot` binary, unrelated)
• CI passes

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